1. Field of the Invention
The present invention relates to apparatus for applying time base corrections to video signals.
2. Description Relative to the Prior Art
In transmitting video signals, particularly where record/playback operations are included, one series problem is that of maintaining a time base consistent with a standard format for video signals, for example, the conventional NTSC signal format. Time base departures from a standard, say the 63.5 .mu.sec. period for one line of a video frame of the NTSC standard, may occur over a wide range of frequencies and are generally referred to as signal flutter.
TV sets are typically adapted to tolerate lowamplitude flutter at low frequencies by including circuitry for loosely slaving the display to the time base of the signal. (To lock the display tightly to the signal time base would introduce objectionable instabilities.) Also, phase-lock-type speed controls on record/playback systems can be designed to eliminate, from a practical standpoint, low frequency flutter, e.g. flutter at frequencies well below the video field frequency. Higher frequency flutter, which may result from such causes as recording-tape-stretch variations and signal-transmission-time variations, proves more difficult to avoid. Such flutter, moreover, is not easily compensated for in the signal processing of a TV set, and can result in picture distortions or other irritating display problems.
One solution to the higher frequency flutter problem, in the case of digital video signals (including of course analog signals converted to a digital form), is to store coded signal components in a digital storage buffer at the incoming rate and then "retrieve" such components in the same sequence but at the standard rate. The storage buffer then absorbs the short time variations by, in effect, expanding and compressing in length in accordance with the differentials in signal rates (see e.g. U.S. Pat. No. 3,860,952). This approach, while effective, is expensive and is best suited to use in video broadcast studios.
A defluttering approach more suited to consumer video systems employs analog shift registers to remove flutter. With this approach video signals are typically read into the registers, one-line-per-register, at the incoming signal rate. Two or more registers receive the signals sequentially and the signal information is read out at the standard rate in the same sequence (see e.g. U.S. Pat. No. 3,931,638). While such an approach is effective for time base correction it does tend to require a great deal of clocking signal and video signal switching. See, also, Fairchild Journal of Semiconductor Progress, Vol. 3, No. 5, Sept, Oct. 1975, pages 16, 17, which discloses a defluttering technique employing input and output registers which sandwich a gate structure adapted for parallel transfer of signals from the input register to the output register.